Heat treatment of alloy steel parts

ABSTRACT

A method of heat treating a high-chromium alloy steel part comprises maintaining the part in a fused salt mixture of cyanides and cyanates of potassium and sodium and controlling the mixture so as to maintain a desired ratio of sodium and potassium and desired proportions of cyanide and cyanate radicals.

United States Patent 1191 Wells Aug. 21, 1973 [54] HEAT TREATMENT OF ALLOY STEEL 2,927,875 3/1960 Muller 148/15.5 PARTS 3,208,885 9/1965 Muller 148/155 3,615,908 10/1971 Waterfall 148/155 [75] Inventor: Kenneth Howard Wells, Solihull,

England FOREIGN PATENTS OR APPLICATIONS 21,868 10/1963 Japan 148/15.5 [73] Ass'gnee- 9 Lucas (lnlusmes) 1,013,712 12/1965 Great Britain 148/155 Blrmmghflmi England 1,052,668 12/1966 Great 1311mm l48/15.5 [22] Filed: June 1, 1971 OTHER PUBLICATIONS [21] Appl. No.: 148,747 Metals Handbook, 8th Ed., V01. 2, 1964, pub. by ASM,

pp. 129-132 & 146-149 [30] Foreign Applinmm, Priority Dam Primary Examiner-Charles N. Lovell Mar. 31, 1971 Great Bntam 8,362/71 Atmmey Hlman & Stem [52] [1.8. CI. l48/l5.5 511 1m. 01. C236 9/14 [57] ABSTRACT [58] Field 61 Search 148/15, 15.5, 28, A method of heat treating a high-chromium y steel 143/123 22 part comprises maintaining the part in a fused salt mixture of cyanides and cyanates of potassium and sodium 5 R f r n i d and controlling the mixture so as to maintain a desired UNITED STATES PATENTS ratio of sodium and potassium and desired proportions 3,317,357 5/1967 Muller 148/155 of cyanide and cyanate liaqlcals' 1,967,994 7 Claims, 6 Drawing Figures 7/1934 Malcolm 148/155 HEAT TREATMENT OF ALLOY STEEL PARTS This invention relatesto a method of nitriding parts formed from alloy steels.

According to the invention a method of heat treating a high-chromium alloy steel part comprises maintaining the part in a fused salt mixture including cyanides and cyanates of potassium and sodium at a temperature between 560C and 580C and controlling the mixture such that the ratio of potassium to sodium is between 2:1 and 1:1 and that cyanide and cyanate radicals are present as follows:- Cyanide radicals up to 20 percent Cyanate radicals not less than 10 percent by weights of the active components of the mixture.

The invention also resides in a process as above described when applied to a part formed of an alloy steel having percentages by weight of the following constituents.

A method according to the invention will now be described by way of example.

A fused saltmixture is prepared having the following constituentsz- Sodium Cyanide Sodium Cyanate Potassium Cyanide Potassium Cyanate Sodium Carbonate Potassium Carbonate The ratio of potassium to sodium being between 121 and 2:1 and the percentages of active components of the bath are within the ranges: cyanide radicals upto 20 percent and cyanate radicals not less than lpercent. The salt mixture is maintained at a temperature of between 560C and 580C.

Specifically the percentages of active compounds are:

Cyanide radicals 14 to 17 percent Cyanate radicals 21 to 25 percent The prepared mixture is placed in an alloy container having a high nickel content.

The method of the invention is particularly applicable to the treatment of spur gears for gear pumps. A spur gear is machined from a percent chromium steel in the fully softened condition. The steel includes the following constituents by weight:

Carbon 6-0 42 0 3 Silicon 0.85-1.15 Chromium 4.75-5.5 Vanadium 0.9-1.1 Molybdenum 1.25-1.7'5 Manganese 0.4 maximum Sulphur 0 03 maximum Phosphorus 0.03 maximum between 30F and 35F. In this last case it is necessary to copper plate the gear all over before the hardening process is started.

The preferred process for copper platingincludes the steps of vapour degreasing, cleaning, etching, applying a 0.0003 inch copper flash in an alkaline bath and subsequently acid copper plating to a required thickness.

When the gear has reached the required temperature (of from 1,040C to 1,060C) it is then cooled either in oil or in an inert gas stream. Substantially immediately thereafter, and preferably while the gear is still warm, it is heated in air for 1 hour at a temperature of between 575C and 585C. This last-mentioned heating takes place in a vertical forced air circulating furnace. The gear is then chilled for a minimum of 1 hour at a temperature of between 6 0C and C and subsequently tempered for 1 hour at between 575C and 585C. The last heating operation also takes place in a vertical forced air circulating furnace.

The hardened and tempered gear is subjected to a finishgrinding operation and then cleaned by degreasing in trichloroethylene vapour and subsequently blasting with a water/air mixture in which is a suspension of white bauxilite micro powder size 400.

The gear is then nitrided by being placed in the fused salt mixture previously described for a minimum of 30 hours. The constituents of the salt mixture are at all times controlled so that the potassium to sodium ratio and the percentages of cyanide and cyanate radicals remained within the limits described above. This control is effected by periodic testing of samples of the mixture, and subsequent additions of saltsas required to maintain the mixture within the prescribed limits. There is no forced supply of oxidanttothe mixture, the reactions taking place as a result of natural oxidisat idii.

A sample removed from the salt is dissolved in water and the carbonate present precipitated with a calcium nitrate solution. After filtration an aliquot portion of the solution is titrated with a standard silver nitrate solution totwo end points. The first end point,using meta cresol purple indicator gives the cyanide content and the second endpoint using potassium chromate gives the cyanate content.

A further sample is converted to chlorides of potassium andsodiumby reactionwith concentrated hydrochloric acid. The chlorides are dissolved in distilled water and an aliquotportion taken to which is added a solution of potassium chloride so that the final solution contains parts per million of potassium. The sodium content of the solution is found by atomising in an air/acetylene flame. The sodiumchloride content of the mixed chlorides can be evaluated and thus the amount of potassium chloride determined. The concentration of sodium and'potassium inthe original sample is thereby computed.

For asalt bath of known dimensions the quantities of sodium or potassium salts required to be addedto the mixture to maintain it within the specified limits can be determined, an equal quantity of salt being removed from the bath prior to the addition of fresh salts. Conveniently additions of salt are made at four-hourly intervals, the quantity and type of salt added on each occasion beingdetermined by a sample taken 1 hour previously. FIGS. 1 to 6 show graphically first approximations of the quantities of-the various salts required to be addedto a bath whose dimensions are 15.75 inches diameter X 18 inches deep to maintain optimum percentages of cyanide and cyanate radicals.

The graphs are shown plotted for a number of constant values of the cyanate radical against varying values of the cyanide radical, both the said values being those determined from the bath samples. The quantities of salt indicated by FIGS. 1 to 3 are added if it is required to raise the sodium content of the bath, and the quantities indicated by FIGS. 4 to 6 if it is required to raise the potassium content. For example, if the sample indicates a cyanate percentage of 22 percent and a cyanide percentage of 14.4 percent the sodium content of the bath being low, the following quantities of salts are added:

Sodium cyanate 6 lb. (FIG. 1) Sodium cyanide 2'): lb. (FIG. 2) Sodium carbonate mil (FIG. 3)

A further sample is taken after adding the salts, to determine the amount, if any, by which the bath constituents differ from the optimum. The differences from the optimum may be due to inaccuracies in the sampling process, intensity of working of the bath, constituents of the parts being treated, etc.

The carbonate content of the bath is determined from a sample by precipitation by the addition of a calcium nitrate solution, the carbonate being filtered out. After thorough washing with hot water, the precipitate is diluted and titrated, with dilute hydrochloric or sulphuric acid, using methyl orange indicator. When the bath has been in use for some time it may be necessary to lower its temperature to between l0C and 530C and to remove the carbonate-rich sludge in order to maintain the required cyanate level. After removal from the bath the gear is allowed to cool in air. Salt residues are removed withboiling water followed by scrubbing in warm water to which a detergent has been added. The white surface layer resulting from the nitriding process is chemically removed and all trace of the chemicals used in this process are themselves removed. The gear is then subjected to a further blasting with the water/air/bauxilite mixture described earlier. If at any stage in the treatment of a gear it is required to store the gear for any significant period, the gear is coated with a lanolin-based dewatering oil.

Components formed of analloy steel. of the kind described and treated by the foregoing process have been found to possess the following characteristics:-

Total etched case depth Izod impact Vickers hardness No:

Gears treated according to the method described have been found to operate satisfactorily in gear fuel pumps for petrol, the said pumps working at pressures up to 1500 pounds per square inch and temperatures up to C. In these conditions there is effectively no lubrication being provided by the fuel. Wear on the gears is nevertheless within acceptable limits.

I claim:

l. A method of nitriding a high-chromium alloy steel part comprising the steps of maintaining the part in a fused salt mixture including potassium cyanide, potassium cyanate, sodium cyanide and sodium cyanate at a temperature between 560C and 580C for a time sufficient to effect the desired nitride case, and controlling the mixture such that the ratio of potassium to sodium is between 2: 1 and 1:1 and that cyanide and cyanate radicals are present as follows:-

Cyanide radicals 14 to 17 percent Cyanate radicals 21 to 25 percent by weights of the active components of the mixture.

2. A method as claimed in claim 1 the part is maintained in the mixture for approximately 30 hours.

3. A method as claimed claim 1 in which the fused salt mixture also contains Sodium Carbonate and Potassium Carbonate.

4. A method as claimed in claim 3 in which control of said mixture includes periodic testing of samples taken from the mixture and subsequently adding to the mixture sodium and potassium salts in quantities based on the samples so as to maintain the values of the constituents of the mixture within specified limits.

5. A method as claimed in claim 4 in'which said sainples are tested at four hourly intervals.

6. A method as claimed in claim 4 in which said quantities of salts are added 1 hour after taking the samples on which said quantities are based. I

7. A method as claimed in claim 4 which includes removing the mixture a quantity thereof equal to the total of the quantities of salts which are added.

I III I 

2. A method as claimed in claim 1 the part is maintained in the mixture for approximately 30 hours.
 3. A method as claimed claim 1 in which the fused salt mixture also contains Sodium Carbonate and Potassium Carbonate.
 4. A method as claimed in claim 3 in which control of said mixture includes periodic testing of samples taken from the mixture and subsequently adding to the mixture sodium and potassium salts in quantities based on the samples so as to maintain the values of the constituents of the mixture within specifiEd limits.
 5. A method as claimed in claim 4 in which said samples are tested at four hourly intervals.
 6. A method as claimed in claim 4 in which said quantities of salts are added 1 hour after taking the samples on which said quantities are based.
 7. A method as claimed in claim 4 which includes removing the mixture a quantity thereof equal to the total of the quantities of salts which are added. 